plan rank indexes in the cascades planner

[normen662]

The heuristic planner plans a rank(score by groups) filter by use of ranking indexes. This issues addresses the planning of rank() in the Cascades planner.

The following features should be supported:

• basic BY_RANK index scans
• index intersections, fetch deterrence, etc. of basic rank index, i.e. the integration into the rest of the planning ecosystem
• proper modeling of the index aggregate functions, i.e. the residual application of rank() as well as the score_for_rank() functions